The present invention is particularly applicable for detecting a low level of fluid in the various portions of a motor vehicle, such as the radiator, oil pan, transmission housing, differential, master brake cylinder, etc. and it will be described with particular reference thereto; however, it should be appreciated that the invention has much broader applications and may be used for detecting low level in various liquid containing receptacles.
In the automotive field, a substantial amount of effort is being devoted to protecting the engine of a vehicle even when the vehicle is subjected to adverse conditions. One condition of general concern is unattended lack of sufficient liquid in various compartments or receptacles of a motor vehicle. If the level of liquid in certain receptacles becomes relatively low it is necessary to immediately replenish the liquid. Sufficient attention to this matter has become more difficult with the advent of self-service gasoline filling facilities. A driver may not recognize the necessity for inspecting the level of certain liquids on a periodic basis. In addition, motor vehicles are becoming more sophisticated and there is a desire to have warning lights on the dashboard for indicating areas of possible concern. These factors and others have contributed to the need for a satisfactory device to detect the level of liquid in various vehicle receptacles, especially the oil pan below the crank case. This oil level is very difficult to monitor. As soon as the engine operates, oil is splashed by the rotating crank and the reciprocating connecting rods. This splashing and agitation can cause false readings on oil level. This is especially true in automatic detectors which have sensing devices within the oil pan for sensing low levels.
There is thus a need for a device which can detect low level of liquid, such as the oil in an oil pan, without being subjected to false readings due to splashing of the oil within the pan. Certain devices have been suggested for detection of low liquid levels. One of these devices is found in SAE publication 800127 by R. W. Tarpley et al entitled Solid-State Automotive Liquid Level Sensing System. In this article, a small silicon chip is used as the probe in a liquid container. As shown in FIG. 4 of this article, the silicon chip has a positive temperature coefficient so that as current is passed through the chip by applying a regulated voltage, the output voltage decreases. As the temperature of the chip increases, the resistance of the chip increases. This causes a corresponding increase in the voltage drop across the strip. Consequently, a reference level in the disclosed system decreases as the voltage decreases with an increase in voltage across the chip. If this measured voltage decreases to a greater extent than the reference voltage decreases, there is an indication that the chip is in air. This means low liquid level. If the chip is in oil, the decrease in voltage does not decrease to the continuously decreasing reference voltage. This indicates sufficient liquid level in the receptacle being monitored since the liquid around the chip dissipates any generated heat and keeps the chip cooled. This particular system has several limitations. First, this chip is difficult to assemble and regulate. The unit has a sharp demarcation. It will detect the absence of liquid at only the chip itself. This makes external adjustment of the probe very difficult and calibration of the system after assembly is not possible.
Another arrangmenet is illustrated in U.S. Pat. No. 4,283,719 wherein a probe is employed with a control circuit to detect low level by a differentiating circuit having a time constant so that the voltage sensitive means detects the rate of change of the voltage signal across the probe. This unit requires a relatively complex control circuit utilizing a continuous current flow through the probe between the positive bus and negative bus of the vehicle. This circuit can be modified to provide continuous monitoring or a single test at the start of the engine. In each instance, two separate circuits are required. One for maintaining the current flow through the probe and the other for detecting level with a probe.
In view of this background, there is still a need for a simplified arrangement which has universal application to various receptacles on the motor vehicle and which can detect a low liquid level accurately and with a minimum of circuit components.